Concrete is a mechanically strong artificial rock-like material that is the most widely used construction material in the world. However, due to the heterogeneity of the composition of its principal components, cement, water and a variety of aggregates, the properties of the final products can vary widely. Concrete is characterized by a high compressive strength, but it also has a relatively low-tensile strength. Its lack of tensile strength can be compensated for through the use of reinforcements (e.g., steel rebar) that can increase the concrete's resilience to tensile forces. Even when reinforced, however, concrete materials can crack as a result of applied structural loading, shrinkage, and thermal deformations, any of which are practically inevitable and often anticipated in restrained conditions. Corrosion of the concrete reinforcements and crack formation are major causes of degeneration, which consequently limit the durability and lifetime of a concrete structure. For example, the presence of cracking reduces the load capacity and stiffness of a concrete structure. Cracks also provide pathways for the penetration of ions that can cause concrete to deteriorate. Chloride ion, oxygen, and carbonating agents can migrate through cracks and lead to corrosion of reinforcing steel which is the major cause of concrete deterioration world-wide. Therefore, the formation of cracks is a dominant form of damage in concrete materials. Consequently, large amounts of time and money are directed to finding ways to improve the quality and durability of concrete, as well as to finding ways to decrease its manufacturing costs.
Early research performed by White and co-workers focused on the ability of materials to self-heal. [White, S. R., et al., Nature (2001) 409:794-797]. They “report a structural polymeric material with the ability to autonomically heal cracks [that] incorporates a microencapsulated healing agent that is released upon crack intrusion. Polymerization of the healing agent is then triggered by contact with an embedded catalyst, bonding the crack faces [and yielding] as much as 75% recovery in toughness” (see, Abstract). Similar self-healing mechanisms are known in concrete materials. It has been proposed that the primary self-healing process in high performance concrete derives from the formation of calcium carbonate resulting from unhydrated cement particles coming in contact with permeating water carrying dissolved carbon dioxide.
In recent years, there has been an increased amount of research focused on the process of biomineralization and its effect on mechanical property recovery in concrete materials. Biomineralization is a metabolic process that takes place in certain microorganisms and results in the formation of hard structures, surfaces, or scale by combining minerals with organic compounds. Certain microorganisms found in hot springs are known to participate in biomineralization processes, which play an important role in the proper functioning of these geothermal ecosystems. Several researchers have explored the concept of biomineralization in an attempt to develop bio-concrete compositions and materials with self-healing properties. For example, Bang, et al. showed that “[Bacillus pasteurii] immobilized cells exhibited the rates of calcite precipitation and ammonia production as high as those of the free cells” and that the calcite “showed little effect on the elastic modulus and tensile strength of the polymer, but increased the compressive strengths of concrete cubes, whose cracks were remediated with . . . immobilized cells” (see Abstract). Similarly, Rodriguez-Navarro et al. demonstrated that “Myxococcus Xanthus-induced calcium carbonate precipitation efficiently protects and consolidates porous ornamental limestone” and that “new [calcium carbonate] crystals are more stress resistant than the calcite grains of the original stone because they are organic-inorganic composites” (see Abstract). Certain microorganisms have found use in concrete technology such as, for example, cleaning agents for concrete surfaces. [DeGraef, B., et al. (2005)]. Other studies have explored bacterial bio-deposition of calcium carbonate for the treatment degraded limestone. [Dick, J., et al. (2006)].